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In this paper, we present a power adjustment scheme to dynamically enlarge and shrink power coverage to speed up tag identification in an RFID system. By dividing a TDMA frame into time slots, the proposed power adjustment scheme can adaptively increase or decrease the transmission power of a reader. Specifically, due to the contention for a TDMA slot from numerous tags, three states of a slot could exist; they are respectively referred to as successful, collided, and idle states. An adjustment factor based on the three states is designed to dynamically adjust the transmission power of a reader. The design of the adjustment factor considers two different aspects. When the number of idle state far exceeds the number of collided state, the first aspect will enlarge the power such that more tags within the coverage can be concurrently identified. On the other hand, when the number of idle state is much smaller than the number of collided state, the second aspect will shrink the power such that the number of tags within the coverage is significantly reduced. The proposed power adjustment scheme is simulated using NS-3. In the simulation, we design three different topologies which place tags in three distributions, uniform, random, and hotspot. From the simulation results, we demonstrate that the proposed power adjustment scheme can speed up the tag identification and save energy consumption, particularly when a large number of tags are placed in hotspot distribution.

Along with the increasing requirements for facilitating the convenience in our living environments, the technology of RFID (Radio Frequency Identification) has been widely deployed. An RFID system consists of a reader and a large number of tags. There are many existing applications using RFID today, for examples, tagging a vehicle for electronic toll collections (ETC), tagging a passing card for subways, tagging merchandise in supermarkets, etc. In these applications, a reader usually needs to read hundreds or thousands of tags’ ID in a short time. Yet, simultaneously reading more than one tags’ ID may create collisions in a reader system. Thus, in recent years, how to effectively and speedily read tags’ ID form a reader has become an important research subject.

Differentiated by the media access control, previous work on resolving the collisions in an RFID system can be divided into four categories, FDMA (Frequency Division Multiple Access), CSMA (Carrier Sense Multiple Access), SDMA (Space Division Multiple Access), and TDMA (Time Division Multiple Access). In FDMA, multiple channels in terms of different frequencies are used among different readers to avoid collisions. For example, Yu, et al. [

Besides the above mentioned three categories, TDMA noticeably attracts the most attentions to researchers. A TDMA frame consists of multiple slots where a tag uses a slot (considered as a channel) to transfer back its ID to a demanding reader. Yet, collisions may occur when more than one tag pick up the same slot. Most of the researchers in TDMA therefore work on a model to estimate the appropriate number of slots in a frame based on different levels of collisions. For examples, Onat, et al. [

The previous work resolved the collisions in an RFID system using TDMA either by adjusting the size of a frame or by dividing the tags into different groups. In this paper, without paying the cost in frequently varying the frame sizes or aforetime dividing the tags into several groups, we propose a dynamic power adjustment (DPA) scheme to speed up the tag identification time. The proposed DPA increases or decreases the transmission power of a reader based on three states (successful, collided, and idle) of a slot in a TDMA frame. Two aspects are considered in adjusting the transmission power. The first aspect increases the power to read more tags, if the number of idle state far exceeds the number of collided state. The second aspect cut power to reduce the number of readings, if the number of idle state is much smaller than the number of collided state.

The remainder of this paper is organized as follows. In Section 2, an RFID system and the format of a tag’s ID are introduced. In Section 3, we propose a dynamic power adjustment scheme for a reader to speed up the tag identification time. In Section 4, simulation on NS-3 is performed and the results are discussed. Finally, conclusion remarks are drawn in Section 5.

1) Three States of a Slot

As shown in

2) Factor to Adjust the Power

To reduce the collisions, the proposed DPA increases or decreases the transmission power of a reader based on the three states of a slot. In other words, reducing transmission power of a reader will reduce the coverage. As a result, the number of tags to be identified is reduced. On the other hand, increasing transmission power of a reader will increase the coverage, which in turn increases the probability of collisions.

Based on the three states of a slot, a factor (F) to adjust the transmission power is computed as shown in Equation (1), where I denotes the number of idle slot, C denotes the number of collided slot, and S denotes the number of successful slot. Specifically, if F is positive and greater than one, then the transmission power of a reader is enlarged by multiplying F. Notice that the second term in Equation (1) is always greater than one. Thus, if I is greater than C, then F should be greater than one.

F = ( I C ) × ( S S + C ) (1)

Equation (2) is used to compute the transmission power ( P n + 1 ) of a reader at Frame n + 1 from the transmission power ( P n ) of a reader at Frame n. Since any one of the three parameters, S, C, and I, could be zero, we have to design another four parameters, Δ 1 , Δ 2 , Δ 4 , and Δ 4 , as shown in Equation (3), for power adjustment. In Equation (3), m is the total number of slot in a frame and P 0 is the minimum transmission power of a reader. Δ 1 is positive or negative depending on whether F is greater than or smaller than one. Δ 2 is always positive and it is designed when C = 0 and I ≠ 0 . To the contrast, Δ 3 is always negative and it is designed when C ≠ 0 and I ≠ 0 . Finally, Δ 4 is positive or negative depending on whether I is greater than or smaller than C. It is noticed that P n + 1 is no change from P n as long as S ≠ 0 , C = 0 , and I = 0 .

P n + 1 = { P n + Δ 1 , if S ≠ 0 and C ≠ 0 and I ≠ 0 P n + Δ 2 , if C ≠ 0 and I ≠ 0 P n + Δ 3 , if C ≠ 0 and I = 0 P n + Δ 4 , if S = 0 and C ≠ 0 and I ≠ 0 P n , if S ≠ 0 and C = 0 and I = 0 } (2)

{ Δ 1 = ( F − 1 ) × P 0 Δ 2 = I m × I S + I × P 0 Δ 3 = − ( C m × C S + C × P 0 ) Δ 4 = I − C m × P 0 } (3)

We illustrate the proposed DPA with two scenarios, as shown in

Parameter | Definition |
---|---|

S | Successful slots in a TDMA frame |

C | Collided slots in a TDMA frame |

I | Idle slots in a TDMA frame |

F | Power adjustment factor |

P_{0} | Minimum transmission power of a reader |

P_{n} | Transmission power in the n-th frame |

P_{n}_{+1} | Transmission power in the (n + 1)-th frame |

d_{0} | Radius of a reader’s minimum coverage |

d_{n} | Radius of a reader’s coverage in the n-th frame |

d_{n}_{+1} | Radius of a reader’s coverage in the (n + 1)-th frame |

To analyze the proposed DPA in terms of tag identification time, the number of undetected tags, and total energy consumption, we perform simulation using NS-3 with an embedded RFID module. As illustrated in

To analyze different placements of tags, three topologies of tag distribution, uniform, random, and hotspot, are designed in the NS-3 simulation, as shown in

We define the transmission power ( P t ) of a reader in Equation (4) and the energy consumption (EC) of a reader in Equation (5). In Equation (4), P r is the receiving power of a tag, d is the distance between a tag and its reader, and λ is the wavelength for radio. In Equation (5), T is the total operation time of a reader.

Parameters | Settings |
---|---|

P_{0} (Minimum transmission power) | 0.05 mW |

P_{r} (Receiving power) | 3.65 × 10^{−7} mW |

P_{max} (Maximum transmission power) | 0.53 mW |

Λ (Wavelength) | 0.33 meters |

m(Slots in a frame) | 16 or 64 slots |

P t = P r × ( 4 π d ) 2 λ 2 (4)

E C = P t × T (5)

From Equation (4) and by substituting P t = 0.53 mW, P r = 3.65 × 10 − 7 mW, λ = 0.33 meters, we can compute the maximum distance ( d max ) between a reader and a tag, which is about 10 meters. In other words, we assume the maximum coverage of a reader is 10 meters in the simulation.

By considering the three different topologies of tag distribution,

Since a reader in SPSC consistently maintains its transmission power in the maximum level, it will consume too much power. To the contrast of SPSC, we introduce two variation versions of the proposed DPA to significantly save the transmission power of a reader. The first variation is referred to as DPA with proportionally increasing power from the minimum to the maximum (PIP-MIM). The second variation is referred to as DPA with proportionally decreasing power from the maximum to the minimum (PDP-MAX).

Although both PIP-MIM and PDP-MAX can greatly reduce the power consumed in a reader, the later version (PDP-MAX) may not 100% read all the tags, since some tags may remain unidentified between the outer coverage when a larger transmission power is used and the inner coverage when a smaller transmission power is adjusted.

By considering 100 tags placed in the three distributions,

From Equation (4) and Equation (5), we can analyze and compare the energy consumption (EC) among SPSC, PDP-MAX, PIP-MIM, and the proposed DPA. However, it is very straightforward to announce that the EC in SPSC should be the largest one, since the transmission power of a reader always remains in the maximum. Additionally, we have no doubt that the EC in PDP-MAX should be the smallest one, since it proportionally reduces the power from the maximum to the minimum in every frame. Yet, from

By increasing the number of tag from 100 to 300,

In this paper, we have presented a dynamic power adjustment (DPA) scheme for a reader to speed up the tag identification time in an RFID system. The proposed DPA can dynamically increase or decrease the transmission power of a reader based on the three states of a TDMA slot, successful, collided, and idle. One of the major contributions of this paper is right in that without paying the cost of frequently varying the frame sizes and aforetime dividing the tags into different groups, the proposed DPA outer-performs a previous work (SPSC) in reducing the tag identification time. Besides, through NS-3 simulation, we have demonstrated that 1) the proposed DPA can read all the tags under the coverage without any missing as it is compared to a proportionally decreasing power scheme (the PDP-MAX); 2) the proposed DPA consumes significantly less energy as it is compared to a proportionally increasing power scheme (the PIP-MIN).

In the future, the proposed DPA can be extended by considering the possible movement of tags. Since a tag may move randomly in every direction, a hop-spot area may move from inner circle to outer circle or vice versa along with the movement of tags. Therefore, how to systematically predict the movement of tags thus dynamically enlarging or shrinking the power coverage would become possible.

The authors declare no conflicts of interest regarding the publication of this paper.

Sheu, T.-L. and Zhu, J.-X. (2018) Dynamically Enlarge and Shrink Power Coverage to Speed Up Tag Identification in an RFID System. Journal of Computer and Communications, 6, 247-263. https://doi.org/10.4236/jcc.2018.611023